Conveyor systems in which conveyor belts are driven in a spiral path with the belt curving edgewise around a series of vertically spaced loops are commonly used to provide a great length of endless conveyor belt in a relatively small space. One conveyor system of this type is disclosed in U.S. Pat. No. 3,348,659 wherein tension is induced in the spiral belt so that there is frictional contact between the radially inner edge of the belt and a plurality of driving elements. This belt system is then frictionally driven by the driving elements which slidingly engage the radially inner edge of the spiral belt loops. The driving elements move faster than the belt and continuously slide past the belt edge to achieve a frictional drive. When safe operating parameters are exceeded for this type of system, the belt is subjected to excessively high tension which can result in inordinate wear and fatigue failure of the belt, as well as causing damage to the conveyor structure. Moreover, such operation can cause surging of the belt which interferes with its smooth operation and disturbs the products being conveyed.
Another conveyor system of this type is disclosed in U.S. Pat. No. 4,741,430. This system uses a positive drive to propel the belt. The drive includes a cylindrical cage rotatable about a vertical axis and having a plurality of vertical driving bars spaced circumferentially around the cage. The bars have radial driving surfaces arranged in overlapping and abutting relationship with protrusions which are spaced along the radially inner edge of the belt in the spiral loops so as to positively drive the belt.
Positive drive spiral belt systems suffer certain drawbacks. Engagement of the driving surface with the belt surface is difficult to initiate and maintain smoothly. The pitch of the belt changes as the belt enters and leaves the series of spiral loops. This changing pitch can not be compensated for by the drive mechanism. Thus, as belt pitch increases, driving contact is lost allowing the belt to become slack and migrate backward. A decrease in belt pitch causes the belt tension to increase resulting in belt surging, increased wear, fatigue failure and other damage to the system. Such disadvantages are overcome by the present invention.